Although the invention may have other uses such as in the replacement of underground steel conduits of different size and having a variety of uses, the invention was made and will therefore be described in connection with devising an effective manner of removal of steel gaslines extending under paved roads and streets, particularly 4" diameter natural gas mains, and replacing of the gasline with plastic piping.
As such underground steel gaslines age, they deteriorate and begin to leak. Apart from inherent safety considerations, the repair and eventual replacement of such deteriorated lines is expensive and disruptive of normal street activity taking place thereabove. Expense can be significantly reduced by replacing the steel gas main with plastic pipe of the same size but, still, the steel pipe must first be effectively removed. In the past, street excavation and trenching along the length of the deteriorated steel piping has been necessary. It is intended by the present invention to avoid such trenching and extensive excavation when removing the old pipe.
A successful alternative to such trenching and excavating has been devised in connection with the removal of cast iron pipelines, this technique being known as "pipe bursting". The underground pipe is broken apart in situ using a pipe bursting tool pulled or pushed therethrough, the tool also incorporating a spreader which pushes the fragmented pipe remains radially outward into the surrounding soil so that the pipe is effectively, though not actually removed to make way for the replacement pipe. The replacement plastic pipe or, alternatively, a larger diameter plastic pipe liner within which the replacement pipe will be received, is usually attached to the pipe bursting tool so that it is pulled into place as the old pipe is removed. For example, see the impact ram boring tools of U.S. Pat. No. 4,505,302 (Streatfield et al) and U.S. Pat. No. 4,732,222 (Schmidt) which employ pneumatically actuated, so-called "impact moles" of larger diameter than the cast iron pipe to be broken and having radially projecting angular blades which initially impact and cut into, and break the pipe as the mole is pulled and hammered therethrough. Similar disclosures are found in U.S. Pat. Nos. 4,720,211 (Streatfield et al), 4,738,565 (Streatfield et al), and 4,674,914 (Wayman et al).
However, these conventional percussive pipe breaker devices, though successful in replacing cast iron mains, have thus far not been successfully adapted for similar removal of steel gas mains, especially those which have all or some of their pipe sections joined using conventional, so-called Dresser couplings. The reason is that steel pipe has greater hoop strength than does cast iron, is more ductile, and therefore does not shatter and fragment as does cast iron. Even the chain of alternately upward and downward projecting pipe cutter wheels followed by a concentric conical spreader for cutting and separating the pipe into two halves as described in U.S. Pat. No. 3,181,302 (Lindsay) will not remove steel pipe in an entirely satisfactory manner, especially where local reinforcement is provided at the pipe joints by the very formidable Dresser couplings. Other possible techniques are known, such as downhole motors, sawblades, plasma cutters and lasers. However, in addition to their inabilities to satisfactorily perform the desired bursting function in steel pipe as aforesaid, all appear to be unduly complicated in construction, or expensive, or require very high power, or are difficult to operate or maintain. Thus, the task of cutting longitudinally all the way through such steel pipe would be extremely difficult using currently known hardware and methods.